Fix tense: 'proposed' -> 'propose'
Explain how SIIT cannot be detected by the node

As mentioned on the list, the attack vector cannot be limited to IPv4
nodes behind SIIT (e.g., by accepting ICMPv6 PTB with MTU values <1280
for such destinations), as such nodes are indistinguishable from native
nodes when the SIIT function is using a Network-Specific Prefix.